For improving the circulation of automobile traffic and traffic safety, certain radio broadcasting stations known as traffic radio stations have been used for transmitting traffic advice which can basically be received on any radio receiver. In order to make it easier for the driver of a vehicle select the traffic transmitter which is responsible for the region in which he is located, an already widespread information system known by the acronym ARI (Auto Radio Information) is in use in Europe that makes use of three designated frequencies additionally modulated, along with the program modulation of FM traffic radio transmitters operating in the VHF frequency band. This system has already been installed in a number of European countries.
For recognition and picking up of the traffic transmission a 57 kHz carrier is provided which identifies all traffic radio transmissions. For the purpose of recognition of the regional identity, the 57 kHz auxiliary carrier is modulated with one of six possible regional frequencies (referred to by letters A-F). In this manner information is given for recognizing to which traffic region the particular traffic radio transmission pertains.
For recognition of break-in transmissions, a second modulation of 125 Hz on the 57 kHz sub-carrier is utilized during a break-in announcement for identifying a particular traffic announcement that is intended to override other types of reception that may be in progress.
For the evaluation and processing of the above-mentioned three characterizing frequencies a special decoder is needed within the radio receiver and the known system advantageously makes possible the construction of economical traffic transmission decoders which are capable of compulsively bringing to audibility the more urgent traffic information. In this respect a supplementary receiver of the auto radio can search in a scanning mode for the appropriate traffic transmission by reference to transmitter and regional identifications. The vehicle driver is thereafter acoustically informed of traffic advice after recognition of the break-in message identification even if, for example, he is listening to another program or to music from the playing of a cassette.
Because of its many advantages, the above-described system has in the meanwhile become widely adopted. The known system, however, has not yet exploited all the available possibilities. Thus the radio program being listened to at the time by the vehicle driver is necessarily interrupted under the system just described for the duration of the traffic information break, which is often considered to be disturbing. Furthermore, the number of traffic advice items which can be transmitted is limited because of the necessary break-in time and also by the attention span the vehicle driver that may not last adequately in the case of longer traffic information breaks. Additional information breaks in foreign languages that are possible in some cases (for the benefit of transient drivers during vacation time) have magnified the "traffic break" durations.
For further improvement of an optimal traffic radio system, a system has become known through publications describing a traffic radio decoder for processing digital signals. These digital signals represent the traffic advice. They are recieved by demodulation of an auxiliary carrier on which the digital signals are modulated, as described in the publication "Internationales Verkehrswesen", reprint from Issue 5/85; Peter Bragas, Leit- und Informationssysteme im Kraftsfahrzeug--ein Beitrag zur Verbesserung des Verkehrsablaufs und der Verkehrssicherheit, pgs. 2-8.
These disclosures concern the radio data system (RDS) traffic transmission decoder. The system for which this decoder is designed involves the transmission of digital signals utilizing a sub-carrier of the same 57 kHz frequency above-mentioned modulated on a broadcast transmission signal in the VHF range , in which the modulation of the 57 kHz carrier is a double-sideband suppressed-carrier aplitude modulation with biphase-coded data signals. With biphase coding there appear no radio spectrum lines in the neighborhood of the auxiliary carrier, so that compatability of the radio data system (RDS) with the earlier system (ARI) is provided. The two systems can thus actually be combined and transmitted with the same broadcast signal.
The basic RDS concept envisions the transmission "digital storage addresses" or "code words". In the radio receiver, or especially in its traffic transmission decoder, components of messages or sentences of traffic advice are stored at respective addresses ready to be called out, either for visual display or for reproduction by means of a speech synthesizer. In the foreground of this system, therefore, there is not so much the sending of traffic advice as such but rather transmission of digital signals which represent particular pieces of advice, so that along with the RDS system other digital transmissions could in principle come, into consideration. What is significant is principally the recognition that traffic advice is becoming standardized. In spite of its multifarious nature it is capable of being subdivided into specific standard texts. This leads to important advantages in connection with digital signal transmission, since it is now possible to allocate address signals in a simple way to the specific content of the standarized traffic messages and to store these content packets in memory for electronic retrieval. All that is then needed from the traffic radio transmission is merely the transmission of the particular digital address signals within a RDS signal, so that traffic advice is transmitted only in the form of a storage address. At the present time the RDS development is only at the beginning of a technically practicable introduction, however, while the previous system above-described has been installed and used for a long time already. Since RDS has not yet been introduced into practice and the earlier system will not be abandoned overnight but rather will remain in use for a long transient phase, the problem facing the existing models of radio receivers is to make it possible for the user to receive, at his choice either the traffic information of the first system or only RDS traffic information. It is further to be taken into account that even in the future there will be countries in which the previous system will continue to be exclusively used and other countries in which the transition phase leading to the exclusive use of RDS may have extremely long duration. There is accordingly the risk that certain traffic announcements will not reach the listeners and particularly the vehicle driver whenever the radio transmitter used is not designed for the particular system for which the vehicles are equipped.